1. Field of the Invention
This invention relates to the shaping and cooling of glass sheets and particularly to the high speed production of bent glass sheets that are toughened by air quenching, and most particularly, for shaping and heat treating relatively thin glass sheets.
Shaped glass sheets are widely used as side windows, bent panes and rear quarter lights in vehicles such as automobiles or the like. To be suitable for such application, flat glass sheets must be shaped to precisely the fine curvatures dictated by the shape and outline of the frames defining the window openings into which the glass articles are installed. It is also important that the articles in vehicles meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with a clear viewing therethrough in their viewing area.
During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increasing the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of the glass sheet to breakage, tempering also causes a glass sheet to fracture into relatively small, relatively smoothly surfaced fragments that are less injurious than the relatively large, jagged fragments that result from the more frequent breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat glass sheets to the softening point of the glass, shaping the heated sheet to a desired curvature, and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed as one of a series of sheets along a substantially horizontal path that extends through a tunnel-type furnace, a shaping station and a cooling area.
In the furnace, the glass sheets are heated to the deformation temperature of glass. At the shaping station, each glass sheet in turn is brought into alignment between a pair of retracted shaping molds of complementary shape. The molds move toward one another to sandwich the glass sheet therebetween to impart their shape thereto and then retract. The glass sheets are then transferred into the cooling area where tempering medium, usually blasts of air, are imparted against the opposite surfaces of the shaped glass sheet to impart at least a partial temper to the glass. The rate of cooling the hot glass through its annealing range determines how much the glass is tempered.
When glass sheets are gripped near their upper edges by tongs for handling during such treatment, the tongs tend to penetrate and deform the upper portions of the glass sheet in the vicinity of the tong gripping points. This deformation becomes more critical in dealing with thinner glass sheets. Furthermore, the time spent from the end of the heating step to the beginning of the rapid cooling step needed for tempering glass sheets must be minimized, particularly in the case of thin glass sheets having a thickness of 4 millimeters and less as compared with prior art thicknesses of 4.5 millimeters and more, because there is less residual heat in thinner glass sheets so that thin glass sheets tend to cool more rapidly than thicker sheets to below a temperature at which a temper can be induced in the glass on sudden chilling.
Glass sheets have been tempered and have been shaped and tempered by the so-called vertical air wall technique in which the glass sheets are moved in a series of glass sheets through a furnace of tunnel-type configuration, then into a shaping station and finally into a cooling area while each glass sheet is supported in balanced relation in an upright position by flows of hot gas that provide both glass sheet support and conductive and radiant heat. The heat may be supplemented by additional radiation heat sources. Flows of gas may be applied against the bottom surface of the glass sheets to support the glass sheets obliquely to a vertical or horizontal position on the gaseous support. For the purpose of this specification, the term "upright support" encompasses any orientation that is neither horizontal nor nearly horizontal.
Such support for the glass sheets by a gaseous bed avoids the marks that result from using tongs to suspend the hot glass sheets. However, the glass sheet must be supported somehow along its bottom edge both before and after it is shaped by mold engagement. In the past, in order to allow the molds to move toward the aligned glass sheet and to retract, it was necessary to support the lower edge of the glass sheet below the bottom edge of the molds of complementary curvature that engage the opposite glass sheet surfaces. Therefore, the lower edge portion of the glass sheet was never engaged between the molds. Such incomplete engagement of the glass sheet above its lower edge left the shaped glass sheet with a bottom edge portion that did not conform within the precise tolerances necessary for shaped glass sheets to produce a precise fit within the frame of a vehicle in which it is to be installed.
2. The Prior Art
U.S. Pat. Nos. 3,341,312 and 3,341,313 to Robert W. Wheeler and Charles R. Davidson, Jr., both provide apparatus that convey glass sheets in succession through a tunnel-like furnace while heated and balanced by either flows of hot gases or a combination of hot gases and radiant heating while the bottom edges are supported on a series of circumferentially grooved rolls that propel the glass through the various treatment stations.
Each glass sheet, on arrival at a shaping station between a pair of complementary press bending molds, is engaged between the press bending molds for sufficient time for the molds to impress their complementary shapes on the heat softened glass. Then, the molds are retracted and the bent glass begins to move toward a cooling area where its opposite surfaces are chilled as rapidly as possible to impart at least a partial temper to the glass.
U.S. Pat. No. 3,450,517 to Ronald E. Richardson discloses a method and apparatus to reduce damage to press bent glass sheets due to tong pull during press bending. A carriage from which a set of glass gripping tongs is suspended is raised a short distance immediately before a pair of press bending molds engage the glass sheet, then the carriage is lowered to its initial position while the molds engage the glass to relax the tongs. After shaping is completed, the molds retract to permit the shaped glass sheet to hang once more from said tongs. While this invention reduces the severity of defects due to tong pull, it still fails to provide complete elimination of tong marks.
U.S. Pat. No. 3,477,840 to Waldemar W. Oelke, Frank J. Carson, Alfred E. Badger and Thomas B. O'Connell uses a notched guide bar to help support glass sheets of triangular configuration which are balanced in oblique dispositions between a pair of gaseous beds. The notch permits the top triangular edge of a glass sheet to move into a position where it can be in alignment between a pair of press bending molds that shape the triangular shaped glass sheet to its desired curvature.
U.S. Pat. No. 3,595,636 to Richard V. Posney discloses complementary press bending molds having grooved press faces provided with apertures to supply heated gas and/or cool gas at the time that glass sheets are engaged between the molds for shaping. The glass sheets in this patent are gripped by tongs for support from carriages that move through a furnace, into the press bending station and subsequently into a cooling area.
U.S. Pat. No. 3,630,706 to Waldemar W. Oelke, Frank J. Carson, and Alfred E. Badger discloses apparatus for supporting glass sheets along their lower edges for movement in an oblique plane of support through a heating furnace, into a shaping station and then through a cooling area. This patent uses conveyor belts to support the bottom edge of the glass sheet while it is engaged between shaping members.
U.S. Pat. Nos. 3,737,297 and 3,834,885 to Robert G. Frank disclose apparatus that conveys glass sheets while supported along its bottom edge by a series of conveyor belts through a furnace, a movable shaping station, and a cooling area. The movable glass shaping apparatus is designed to simultaneously transport the glass from a furnace exit to a cooling area spaced therefrom while it is being shaped. It is necessary to return the glass shaping apparatus to the vicinity of the exit of the furnace in order to pick up the next glass sheet being produced in order to provide a mass production operation involving a series of glass sheets.
U.S. Pat. No. 3,854,920 to Solomon E. Kay and John Pickard discloses apparatus for shaping glass sheets by the press bending method involving a pair of molds that are heated in such a manner that the mold and glass sheet temperature is substantially uniform at the end of a press bending operation. The glass sheets in this apparatus are suspended from tongs.
In all of the prior art patents enumerated above, the glass sheets undergoing press bending are either engaged by tongs that clamp against the upper edge portion of the glass to impart local deviations from desired curvature or the glass sheets are supported along their edge below the lower edge of the press bending molds during shaping. In the latter case, the bottom edge of the shaped glass sheet fails to conform to the shaped desired because prior art shaping molds never engaged the bottom edge portions of glass sheets.